Etched oil shale fracturing

ABSTRACT

A METHOD FOR FORMING SUBSTANTIALLY HORIZONTAL FRACTURES IN A SUBSURFACE OIL SHALE FORMATION BY CHEMICALLY GENERATING A SUBSTANTIALLY HORIZONTAL PLANE OF WEAKNESS AND THEN HYDRAULICALLY EXTENDING A FRACTURE INTO THIS PLANE OF WEAKNESS.

Sept. 20, 1971 P. J. CLOSMANN ETAL 3,605,889

ETCHED OIL sum. rnAcT'uRINe Filed May 13, 1970 INVENTORS:

P.J- CLOSMANN C T. DEEDS M.H. WAXMAN THEIR ATTORNEY FIG.

United States Patent Office Patented Sept. 20, 1971 3,605,889 ETCHED OIL SHALE FRACTURING Philip J. Closmann, Charles T. Deeds, and Monroe H.

Waxman, Houston, Tex., assignors t Shell Oil Company, New York, N.Y.

Filed May 13. 1970, Ser. No. 36,856 Int. Cl. E21b 43/27 vs. Cl. 166-254 7 Claims ABSTRACT OF THE DISCLOSURE A method for forming substantially horizontal fractures in a subsurface oil shale formation by chemically generating a substantially horizontal plane of weakness and then hydraulically extending a fracture into this plane of weakness.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a method for fracturing subsurface earth formations. More particularly, it relates to a method for forming horizontal fractures in a subsurface oil shale formation to facilitate the in situ recovery of hydrocarbons therefrom.

DESCRIPTION OF THE PRIOR ART Large subterranean deposits of oil in the form of oil shale are found in various sections of the United States and Canada. Among the methods, suggested for recovering oil from these oil shale deposits is the in situ treatment of the oil shale according to chemical or thermal methods. However, these methods are often difiicult or impractical to use because the generally impermeable nature of the oil shale deposits makes it difficult to apply heat or chemicals to any areally extensive portion of the formations. Increasing the permeability of oil shale deposits by conventional hydraulic fracturing procedure has proven generally ineffective because in many oil shale formations hydraulic fracturing produces vertical fractures which, in comparison to horizontal fractions, are inferior with respect to the distribution of fluid over a region having a significant areal extent and with respect to the interconnecting of a pair of wells.

SUMMARY OF THE INVENTION It has been suggested that horizontal fractures may be obtained by overpressuring rock in the vicinity of a plane of weakness. The present invention provides a method for forming horizontal fractures in an oil shale formation by continuously generating a substantially horizontal plane of weakness into which a fracture is extended.

The plane of weakness is generated by contacting the formation with a reagent or sequence of reagents which attack at least some of the rock matrix of the formation. Preferably, the plane of weakness is formed by contacting the oil shale formation along a seam between a relatively kerogen rich layer and a relativel'yikerogen deficient layer of the deposit with a sequence of reagents capable of removing silicate and carbonate minerals as well as kerogen from the formation. A fracture is continuously extended into the plane of weakness by adjusting the fluid pressure at the face of the formation to provide a pressure sufficient to overcome the vertical compressive stresses along the plane of weakness.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a vertical sectional view of a subsurface oil shale formation traversed by a well which is equipped with apparatus suitable for the practice of this invention.

FIG. 2 is a vertical sectional view of an oil shale formation 'which has been treated according to the teachings of an embodiment of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to FIG. 1, we see a subsurface oil shale formation 10. This formation 10 comprises a rock matrix which is composed at least in part of compacted very fine grained particles (not shown) which may be particles of organic or inorganic matterfor example, a common variety of oil shale may include, among others, fine grained particles of silicate minerals such as quartz or feldspar, fine grained particles of carbonate minerals such as limestone or dolomite, and organic matter in the form of kerogen. A given oil shale deposit may contain many layers, or strata, of oil which vary in composition. Some layers may be relatively rich in organic "material in comparison to other layers. To illustrate this, the oil shale formation 10 of FIG. 1 is shown as having two layers, a layer 11 relatively rich in kerogen and a layer 12 having a relatively low kerogen content. The layers 11 and 12 meet at a boundary or seam 13.

To form a substantially horizontal fracture in the oil shale formation 10 according to the method of this invention, one may -extend a well borehole 14 into the formation '10 and then determine the location of zones rich in kerogen, such as layer 11, and of seams separating relatively rich zones from zones having a lower kerogen content, such as, seam 13, by methods well known in the art, such as by core analysis. The well may then be completed as by extending a tubular casing 15 into the borehole and fixing the casing 15 in place with a cement 16.

A notch isr'then cut through the casing 15 and cement 16 and into the formation 10, preferably from a point in the borehole 14 substantially opposite the seam 13 between the layers 11 and 12 of varying kerogen content. The notch 17 is preferably a substantially horizontal one which extends into the formation 10 around the full circumference of the borehole 14. It may be formed by any of a number of procedures well known in the art, but preferably is formed by jetting a fluid containing an abrasive material, such as silicon carbide or Alundum, suspended in a jetting liquid through a jetting means such as nozzle 18 which is attached to a string of tubing 19 which is rotated as the jetting fluid is pumped down the tubing 19 and through "the nozzle 18 from a pump means (not shown) to c ut the circular notch 17.

After the notch 17 has been extended. into the formation 10 by cutting through the casing E5? and cement 16 in a conven a1 manner, the boundaries of the notch 17 may be furtl') extended into the formation 10 with the jet means by iising an abrasive jetting fluid preferably alternately containing, in addition to an abrasive material, a first jettingyliquid comprising a reagent or mixture of reagents capable of dissolving at least part of the inorganic portion ofthe rock matrix and a second jetting liquid comprising a reagent or mixture of reagents capable of attacking the organic portion of the oil shale to at least partially remove kerogen therefrom. These reagents contribute to the extention of the notch by chemically etching at least a portion of the formation 10. For example, in an oil shale formation 10 containing silicates and carbonates, a mixture of hydrofluoric acid (HF), which is effective in dissolving silicates, and hydrochloric acid (HCl), which is effective in dissolving carbonates, may be chosen to selectively dissolve part of the inorganic portion of the rock matrix. A mixture of HF and HCl may be introduced as a vapor phase, thereby increasing the total pressure in the system. This tends to increase the vapor pressure of soluble organic bitumens thus aiding in their removal. The HF-HCl mixture may expose kerogen which may thereafter be attacked by contacting the formation with a reagent or mixture of reagents from the group of reagents capable of dissolving and/or swelling kerogen (such as chloroform or any of the series of halogenated homologs, a mixture of acetic acid and benzene, phenol, phenol, cresol, etc.). The result of such a treatment is to expose additional inorganic portions of the rock matrix which may then be dissolved by again contacting the formation with the HF-HCI mixture. The abrasive jetting-chemical etching procedure may be continued for a period sufficient to extend the radius of the notch 17 to the maximum radius at which the abrasive action contributes significantly to the rate of extention of the notch. When this point is reached, the abrasive agent may be eliminated from the fluid pumped down the tubing 19.

The notch 17 may be further extended into the formation by continuing to alternately contact the walls of the notch with the HF-HCl mixture and with a reagent capable of dissolving and/or swelling kerogen. Repetition of this process leads to the continuous chemical generation of a plane of weaknessa plane in which at least a portion of the formation is etched away. The chemically induced plane of weakness is biased toward extention in a substantially horizontal plane because it extends from the substantially horizontal notch 17 and because the chemical etching process may be particularly effective along a seam, such as substantially horizontal seam 13, between a kerogen rich layer 11 which may be relatively more susceptible to attack by a kerogen solvent and a kerogen deficient layer 12 which may be relatively more susceptible to attack by the HF-HCI mixture.

The desired horizontal fracture may be continuously extended into the chemically induced plane of weakness by adjusting the pump means (not shown) to provide a pressure in the borehole 14 adjacent the formation which is sufficient to overcome the vertical compressive stress along the plane of weakness. This vertical compressive stress may be determined by methods well known in the art. In many instances, it is substantially equal to the total weight per unit area of the overlying formations.

FIG. 2 shows .a Well equipped with apparatus of a type which may be employed to alternately contact the walls of a notch 17 of relatively large radius with a HF-HCl mixture 20 and with a chloroform 21, or other kerogen solvent having a density greater than the density of the HF- HCl mixture 20. A flow path is provided from a pump means 31 to a point in the borehole 14 below the notch 17 as by lowering a string of tubing 22 having one or more perforations 23 therein into the borehole 14 so that the perforations 23 are positioned below the notch 17. The borehole 14 may then be packed off with a closure means such as a packer 24 below the tubing perforations 23. The casing-tubing annulus 25 is closed to fluid flow above the notch 17 as by' a well head closure means 26. A flow means such as pipe 27 provides a flow path opening into the annulus 25 from a second pump means 32. Valves 28 and 33 provide means for allowing fluid to flow out of the annulus 25 and the tubing 22, respectively.

To extend a horizontal fracture 30 into a formation 10 from a notch 17, the HF-HCI 20 mixture is pumped down the tubing 22, through the perforations 23 and into the borehole 14 in an amount at least sufficient to assure the filling of the notch 17. The notch 17 is then filled with the more dense chloroform 21 by pumping the chloroform 21 down the tubing 22 and into the wellbore 1 4 in a similar manner and amount. Since the HF-HCl mixture 20 is lighter than the chloroform 21, it floats on the heavier liquid and is displaced into the annulus 25 above the notch 17. The walls of the notch 17 may then be alternately contacted with .the HF-HCI mixture 20 and the chloroform 21 to extend a plane of weakness 34 into the formation 10 by adjusting the level of the (HF-HCl)-chloroform interface 29 by properly adjusting the pump means 31 and 32 and the valve means 28 and 33. Concurrently, the fluid pressure in the notch 17 is maintained at a pressure sufficient to open a horizontal fracture 30 along the plane of weakness 34 which is continuously generated by the action of the reagents on the formation by adjusting the pump means 31 and 32 as necessary to maintain a fluid pressure in the borehole 14 adjacent the formation 10 which is sufficient to overcome the vertical compressive stress along the plane of weakness 34. The rate at which the plane of weakness 34 is chemically extended into the formation 10 may be increased by heating the reactive fluids prior to their injection into the borehole 14. Pressure control in the borehole may be facilitated by filling the annulus 25 above the HF-HCl mixture 20 with a nonreactive fluid having a density less than that of the HF- HCl mixture 21, such as kerosen or some low molecular weight liquid hydrocarbon. As the fracture 30 is extended into the formation, the reagents are preferably periodically replenished as by circulating fresh HF-I-ICl mixture or fresh chloroform down the tubing 22.

In summary, the present invention provides a method for forming substantially horizontal fractures in a subsurface oil shale formation at least in part composed of a rock matrix which includes an organic portion and an inorganic portion. The method comprises the steps of extending a borehole into the formation; forming a substantially horizontal notch in the formation at a selected depth in said borehole; chemically extending the notch into the formation by alternately contacting the walls of the notch with a first reagent capable of dissolving at least part of the inorganic portion of the formation and with a second reagent capable of dissolving and/ or swelling the organic portion of the formation to expose additional inorganic matter; packing off a selected interval of the borehole which includes the notch; and pumping a fluid into the packed off interval of the borehole and into the extended notch at a pressure sufficient to open a substantially horizontal fracture extending into the formation from the borehole.

We claim as our invention: 1. A method for forming substantially horizontal fractures in a subsurface oil shale formation at least in part composed of a rock matrix which includes an organic portion and an inorganic portion, the method comprising the steps of:

extending a borehole into the formation; forming a substantially horizontal notch in the formation at a selected depth in said borehole;

chemically extending said notch into the formation by alternately contacting the walls of the notch with a first reagent capable of dissolving at least part of the inorganic portion of the formation and with a second reagent capable of dissolving at least some of said organic portion of the formation;

packing off a selected interval of the borehole which includes said notch; and

pumping a fluid into said packed off interval of the borehole and into said extended notch at a pressure sufficient to open a substantially horizontal fracture extending into the formation from said borehole.

2. The method of claim 1 including the steps of determining the location in said formation of a seam separating a layer of said formation which is relatively rich in kerogen from a layer of said formation which has a lower kerogen content and selecting said selected depth in said borehole at which said notch is formed to be substantially opposite said scam in said formation.

3. The method of claim 2 wherein said second reagent is chloroform.

4. The method of claim 1 wherein said first reagent is a mixture comprising hydrofluoric acid and hydrochloric acid.

5. The method of claim 1 wherein said first reagent is a mixture comprising gaseous hydrogen chloride and gaseous hydrogen fluoride.

6. A method for forming substantially horizontal fractures in a subsurface oil shale formation at least in part composed of a rock matrix which includes an organic portion and an inorganic portion, the method comprising the steps of:

extending a borehole into, the formation; forming a substantially horizontal notch in the formation at a selected depth in said borehole; chemically extending said notch into the formation by alternately contacting the walls of the notch with a'first reagent capable of dissolving at least part of the inorganic portion of the formation and with a second reagent capable of swelling at least some of said organic portion of the formation; packing off a selected interval of the borehole which includes said notch; and pumping a fluid into said packed off interval of the borehole and into said extended notch at a pressure suflicient to open a substantially horizontal fracture extending into the formation from said borehole. 7. A method for forming substantially horizontal fractures in a subsurface oil shale formation at least in part composed of a rock matrix which includes an organic portion and an inorganic portion, the method comprising the steps of:

extending the borehole into the formation; determining the location in said formation of a seam separating a layer of said formation which is relatively rich in kerogen from a layer of said formation which has a lower kerogen content; casing said borehole with a tubular casing; cutting a substantially horizontal notch in said casing at a selected depth in said borehole substantially opposite said seam; extending said substantially horizontal notch into said formation by jetting a fluid into contact with said formation which fluid comprises an abrasive material suspended in a jetting liquid;

alternating said jetting liquid between a first liquid which contains a mixture of hydrofluoric acid and hydrochloric acid and a second liquid which contains a reagent capable of attacking the organic portion of the formation to expose additional inorganic matter;

thereafter, extending a substantially horizontal plane of weakness into said formation from said notch by continuing to alternately contact the walls of said notch with said first liquid and said second liquid; and

continuously extending a fracture into said plane of weakness by pumping said first liquid and said second liquid into said notch with a pump means and adjusting said pump means to provide a fluid pressure in said notch suflicient to open a substantially horizontal fracture along said plane of weakness.

References Cited UNITED STATES PATENTS 2,675,083 4/1954 Bond et al 166307X 3,050,119 8/1962 Fast et al. 166308 3,130,786 4/1964 Brown et al 166298X 3,175,613 3/1965 Houg et al. 166298 3,346,048 10/1967 Strange et al. 166271X 3,500,913 3/1970 Nordgren et al. 166271X 3,501,201 3/1970 Closmann et al. 166271X IAN A. CALVERT, Primary Examiner US. Cl. X.R. 

